Responses of lettuce (Lactuca sativa L. var. longifolia) grown in a closed hydroponic system to NaCl₂-salinity

Lettuce plants (Lactuca sativa L. var. longifolia) grown in a closed hydroponic system using perlite as substrate were exposed to similar levels of NaCl- and CaCl₂-salinity in an attempt to distinguish between osmotic and salt-specific effects on plant growth and quality. A standard nutrient solution for lettuce (contrl) was compared with two salinity levels and each salinity level was imposed by adding either NaCl or CaCl₂ to the standardc nutrient solution. The mean electrical conductivity in the root zone, as indicated by the values measured in the drainage solution , was 2.5, 3.7, 3.5, 5.6 and 5.4 dS m-1 in the control, the low NaCl-, the low CaCl₂, the high NaCl- and the high CaCl₂-salinity treatment, respectively. The increasof NaCl-salinity in the root zone restricted the shoot growth of lettuce. In contrast, the CaCl₂-salinity had no significant effect on the aboveground growth of the plants. The root growth was not affected by any salinity treatment. The leaf Cl concentration was increased in all salinity treatment but the NaCl-salinity resulted in significantly higher Cl levels in leaf compared to Ca Cl₂-salinity at similar Cl concentrations in the root zone. The leaf Mg concentration was decreased in all salinity treatment but the most marked suppression was observed at the highest CaCl₂-level. The leaf K was restricted by salinity without significant differences between salinity levels and source. The leaf chlorophyll and asorbic acid concentrations were not affected by either of the salinity treatments. It was concluded that the effects of salinity on lettuce are primarily Na-specific.